I suppose this is possible, but it will be very difficult. You will need to create a child theme and reset each and every width from a pixel specification (px) to a percentage specification (%). However, because all browsers display things differently, it will still not look the same on everybody’s computer. It might look fine on yours, but weird on someone else’s. For example, if you have a width:100% specification, it might look fine on your monitor (800×600), but for someone with a very wide browser (over 1200px wide, for example), they are going to end up with very wide lines of text which can be incredibly difficult to read. If people have trouble reading your site, they move on.
You will be able to change the width of some features with the next update (1.6), but believe me, you want to stick with pixel specifications and be very careful with it.